RFID “tags” can be separated into two broad categories: active tags and passive tags. Active tags are characterized by a local power source such as a battery. Active tags generally transmit information by broadcasting on an RF carrier frequency of choice using a locally generated RF carrier. Active tags are typically used to transmit over long distances, often referred to as “far field communications” (FFC). Antennas used with active RFID tags tend to be large to allow for the communications over long distances.
Passive tags are not powered. Passive tags derive the energy needed to power the tag from an interrogating RF field, and use that energy to transmit response codes by modulating the impedance that the antenna presents to the interrogating field, thereby modulating the signal reflected back to the reader antenna. Passive tags are typically used to transmit over short distances, often referred to as “near field communications” (NFC). For example, passive tags operating at 13.56 MHz are typically designed to communicate with RFID readers a few centimeters away.
Passive tags are typically connected to “loop antennas.” One example of a loop antenna is shown in U.S. Pat. No. 6,568,600, issued to Carpier et al. on May 27, 2003 (the '600 patent). The device described in the '600 patent is recognizable as a “credit card sized” passive RFID card (more specifically, a card that conforms to ISO 7816 size requirements). The loop antenna is necessarily large because passive tags are powered using energy received by the antenna from signals transmitted by the RFID reader.
FIG. 12 shows a power supply voltage developed over time by rectifying a voltage induced in a loop antenna in the presence of an interrogating RF field. Once the power supply voltage reaches a critical value, the tag is powered up and can operate. As the antenna size is reduced, it takes longer for the power supply voltage to reach the critical value, and the tag operation may not meet response time specifications. Below a certain antenna size, the power supply voltage may never reach the critical value, and the tag may never power up.
Antenna design for RFID applications is described in a Microchip Technology, Inc. application note entitled “Antenna Circuit Design for RFID Applications” by Youbok Lee, Ph.D., published in 2003 (no month given). Dr. Lee's application note describes in great detail how to determine size requirements for a passive RFID tag antenna to operate at 13.56 MHz. On page 5 of the application note, Dr. Lee shows that the optimum radius of the loop antenna coil is equal to 1.414 times the required read range. This analysis confirms that for a read range on the order of a few centimeters, a credit card sized loop antenna can be made near optimal.
Passive tags are seeing widespread use in many applications. For example, mobile device manufacturers are embedding passive RFID tags in mobile devices for NFC applications. Example mobile applications include, but are not limited to, ticketing and mobile payments. U.S. Pat. No. 7,333,062 issued to Leizerovich et al. on Feb. 19, 2008 (the '062 patent) shows a mobile phone with an integrated loop antenna for an NFC device. As shown in the '062 patent, the mobile phone provides the real estate necessary to implement a loop antenna at 13.56 MHz.
There have been attempts to implement passive tags in smaller mobile devices. These attempts have met with limited success due in part to the size of the loop antenna. For example, FIG. 13 shows an RFID tag implementation in a secure digital (SD) memory card manufactured by Wireless Dynamics, Inc. of Calgary, Alberta Canada. Card 1300 includes an antenna, but the SD card is significantly oversized as a result. Also for example, U.S. Patent Application Publication No.: US 2006/0124755 A1 shows a memory card having a passive tag, but the card must be inserted into a slot to access a loop antenna on a different device. In this implementation, mobile device real estate is still relied upon for loop antenna implementation. It can be seen, therefore, that the size of antennas are proving to be a barrier to further miniaturization of passive RFID tags.
FIG. 14 shows a prior art smartcard controller and antenna in combination. Smartcard controller 330 includes a contactless interface that includes two pads 1472 and 1474 intended for connection to a coil (antenna 1480). Smartcard controller 330 also includes bridge rectifier 1420 to rectify an alternating voltage present on pads 1472 and 1474 when antenna 1480 is inductively coupled to another device and in the presence of an interrogating RF field. Capacitor 1440 is typically tuned to create a resonant circuit at the frequency of interest (e.g., 13.56 MHz). When antenna 1480 is a large loop antenna, then bridge rectifier 1420 provides power to internal circuits as shown in FIG. 12. Demodulator 1430 demodulates data present in the interrogating RF field, and load modulation driver circuit 1410 modulates an impedance seen by the device presenting the interrogating RF field when the coil (antenna 1480) is inductively coupled to a separate device that is presenting the interrogating RF field. This creates a half-duplex communications path between the device presenting the interrogating RF field and smartcard controller 330. Examples of smartcard controllers are the “SmartMX” controllers sold by NXP Semiconductors N.V. of Eindhoven, The Netherlands.
A need exists for a small footprint RFID tag that does not rely on an external device to house an antenna. A need also exists for a memory card compatible RFID tag that is compatible with standard memory card slots on mobile devices.